To form an oil or gas well, a bottom hole assembly (BHA), including components such as a motor, steering assembly, one or more drill collars, and a drill bit, are coupled to a length of drill pipe to form a drill string. Tools including electronic instrumentation are typically positioned on the BHA to obtain measurements of the downhole environment while drilling. Once assembled, the drill string is then inserted downhole, where drilling and data collection by the tools commence.
The downhole tools and their associated electronic instrumentation must be able to operate near the surface as well as many thousands of feet below. Since temperature within a wellbore tends to increase with increasing depth, the tools may be subjected to severe thermal loads, depending on the depth of the wellbore. Moreover, during drilling, the tools experience vibrational loads due to operation of the drill bit and pressure loads from drilling mud passing through and around the drill string. In some circumstances, the tools are exposed to wellbore temperatures and pressures exceeding 200° C. (473° K) and 20,000 psi (approximately 138,000 kPa).
The maximum operating temperature limit of electronic instrumentation in the downhole tools can be significantly less than the surrounding wellbore temperature, depending on wellbore depth, and may be no more than 125° C. (398° K). As a consequence, prolonged exposure of the downhole tools to the severe thermal environment of the wellbore may cause the temperatures of the electronic instrumentation to exceed their maximum operating limit, thereby resulting in reduced service life and perhaps failure of the tools.
Servicing or replacement of the downhole tools necessitates the drill string be pulled from the wellbore. Once the tools are repaired or replaced, the drill string is then run into the wellbore again, and drilling may resume. Given the costs associated with interrupting drilling and pulling the drill string from the wellbore, apparatus which prolong the service life of electronic instrumentation included within the downhole tools are particularly desirable.